Resolution properties and 3-D reconstruction from multi-azimuth wide-angle data in the Baltic region

Abstract Recent development of geotomography methods and wider use of seismic array observations have improved conditions for studying the 3-D velocity structure of the earth. The solution to this problem has a number of specific features, which ranges from the typical irregular geometry of the observation sites and seismic sources to the possibilities for seismic tomography at different scales (global, regional and local). The international BABEL survey provided one such spatial array with irregular geometry, many sources and a relatively small number of observation points. The seismic ray coverage is not uniformly dense and sufficient enough for reconstructing the media within the framework of tomography approach. This article presents results of a 3-D velocity reconstruction using data from shots along BABEL lines 1, 6 and 7, recorded by all available land stations. Estimation of the accuracy of reconstruction is made on the basis of mathematical modeling, model parametrization and starting model selection. Modeled velocities of 7.45–7.55 km/s between 46 and 57 km are transitional between those traditionally assigned to lower crust and mantle and are interpreted to represent high grade metamorphic crustal rocks or intermixing of crust and mantle at a seismically small scale (100 m).